1. Field of the Invention
The present invention relates to a variable-gain amplification circuit, a receiver, which makes use of the variable-gain amplification circuit as a high-frequency amplification circuit having an AGC (automatic gain control) function, and a receiver IC.
2. Description of the Related Art
There is known a variable-gain amplification circuit with few small distortions as disclosed in documents such as Japanese Patent Laid-open No. 2005-312016 referred to as Patent Document 1 hereinafter. The variable-gain amplification circuit employs transistors and/or MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). The transistors and/or MOSFETs employed in the variable-gain amplification circuit are connected to each other by use of cascade coupling.
The variable-gain amplification circuit has a configuration like one shown in FIG. 21. For example, the variable-gain amplification circuit shown in FIG. 21 is employed as a high-frequency amplifier in a TV tuner. The high-frequency amplifier has an AGC (automatic gain control) function to make the output signal level fixed for an input signal level higher than a level determined in advance.
In the typical configuration shown in FIG. 21, MOSFETs 1 and 2 are connected to each other by use of cascode coupling. To put it in detail, the drain of the MOSFET 1 is connected to the source of MOSFET 2 and the source of the MOSFET 1 is connected to a ground terminal. The drain of the MOSFET 2 is connected to a power-supply line of a voltage power supply +Vcc through a coil L1 and connected to the terminal of the ground through a variable-capacitance capacitor Cv2. The drain of the MOSFET 2 is also connected to an output terminal.
An input signal is supplied to the gate of the MOSFET 1 by way of a transformer T1, a variable-capacitance capacitor Cv1 and a capacitor C1. The secondary coil of the transformer T1 and the variable-capacitance capacitor Cv1 form a tuning circuit.
An AGC voltage Vagc is supplied to the gate of the MOSFET 2. The gate of the MOSFET 2 is connected to the terminal of the ground through a capacitor Co.
In addition, in this typical configuration, between the power-supply line of the voltage power supply +Vcc and the terminal of the ground, a resistor R1 and MOSFETs 3 and 4 are connected to each other to form a series circuit. To be more specific, the source of the MOSFET 3 is connected to the drain of the MOSFET 4. The gate of the MOSFET 3 is connected to the gate of the MOSFET 2 whereas the gate of the MOSFET 4 is connected to the gate of the MOSFET 1.
In the typical configuration shown in FIG. 21, the AGC voltage Vagc supplied to the gate of the MOSFET 2 is generated by a circuit provided at a stage following this variable-gain amplification circuit. For an input signal level higher than a level determined in advance, the AGC voltage Vagc is reduced. With the AGC voltage Vagc reduced, the voltage appearing between the drain and source of the MOSFET 1 is also reduced, driving the MOSFET 1 to a three-pole tube region. In this region, the gain of the variable-gain amplification circuit starts to decrease and the variable-gain amplification circuit works as a variable-gain amplifier. The variable-gain amplification circuit having a configuration shown in FIG. 21 is known as a variable-gain amplifier having small distortions.